BEIJING  "This is going to be a little shocking," Staci Simonich says as she unscrews the aluminum housing that covers the collection filter in her air-sampling machine atop the geology building at Peking University.

When the environmental chemistry professor pulls the cover off, the filter underneath is as gray as the lint filter in a much-used clothes dryer.

Simonich has been taking air samples for the last seven days. She hasn't finished her analysis and doesn't have exact figures, but her rough estimate is that pollution levels are between two and seven times higher than in a typical larger American city and six times higher than the average during the 1996 Atlanta Olympics, the Oregon State University researcher says.

The International Olympic Committee is monitoring the air hourly and will take action if there is concern, says Arne Ljungqvist, chairman of the IOC medical commission.

Such concern is reasonable, says John Balmes, a pulmonologist and professor of environmental health at the University of California at San Francisco.

Particulate matter is the most dangerous. Burning materials create tiny pieces of carbon, which absorbs various chemicals onto its surfaces. These burn and injure the lining of the airway and lungs, Balmes says.

For people with pre-existing heart and lung disease, exposure to high levels of particulate matter can elevate their risk. If the air in Beijing remains highly polluted, Balmes expects increased heart problems and perhaps deaths, especially among older spectators with heart and lung problems.

Because of concern about pollution, four American cyclists arrived in Beijing wearing face masks. They released a statement Wednesday apologizing for their action, which they called "a precautionary measure."

Another concern is ozone, formed via a chemical reaction of pollutants in the air and sunlight. Ozone can cause decreased lung function, airway inflammation and chemically burns the airways. Studies done during the Los Angeles Olympics in 1984 found that cyclists exposed to ozone in the laboratory had decreased performance.

And because exercising athletes take in between seven and eight times more air than a sedentary person, they're more likely to be affected, Balmes says.

Exactly what the pollution levels in Beijing are is unclear. The official Olympic air pollution figures are posted daily by the Beijing Municipal Environmental Protection Bureau, based on averages of samples taken for a 24-hour period at locations around the city.

Simonich says the values she's seeing are generally about two times higher than what's being reported by the city, though she notes that her collection site at Peking University is more urban. She did say the levels are 20%-40% lower than they were a year ago.

The BBC has been taking handheld particulate matter readings, which have been significantly higher than those published by the city. The British Broadcasting Corp. reported 134 micrograms per meter July 28. The city reported 98, far above the World Health Organization's guidelines for healthy air of 50.

Weather Channel senior forecaster Buzz Bernard has found Beijing visibility at about 3 miles. During the best weather of the last few weeks, when blue skies appeared over the city, it was up to 6 miles.

In an effort to lower the number, China shut down factories, closed cement plants and restricted private vehicle use in the weeks leading up to the Games.

The hope, says Tong Zhu, an environmental science professor at Peking University who oversees air-quality experiments, is for a lasting effect.

"There has been a definite improvement over the last year," he says. "Most of the anti-pollution efforts put into place for the Olympics will stay in effect afterwards," which should go a long way to eliminating Beijing's daytime haze.

The location of the city is also a contributing factor. Beijing is mostly flat, ringed by mountains on its north, east and west sides. It is open to the south, which allows pollution and dust from southern urban and manufacturing areas to flow in, but no way to flow out.

"A perfect day for the Olympics would be rain at night to wash the pollution that's built up out of the air, cloud cover during the day to limit the sunlight that causes chemical reactions and then wind from the north to blow the pollution out of the basin," Simonich says.

The nightmare scenario: no rain and an inversion layer traps pollution, with no wind to clear it away. Showers are predicted this weekend.

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